To evaluate the possibility that abrupt withdrawal of propranolol causes over-activity of the sympathetic nervous system or enhancement of platelet function, we examined sympathetically-mediated heart rate responses, platelet survival and aggregation, and plasma and urine catecholamines in 10 normal subjects before and at various times after propranolol therapy. Preliminary results suggest that propranolol withdrawal may be associated with shortened platelet survival. No consistent change was noted in heart rate responses to direct and reflex sympathetic stimulation.